Solid substrates are often provided with coatings e.g. for protection. As examples of such substrates may be mentioned pipes, tubes, profiles, wires, cables, etc. The coating preferably comprises a polymer composition that is melt coated on the substrate, e.g. by extrusion. Among the desired properties of the polymer coating composition it is particularly preferred that it has good melt coating processability, good environmental stress cracking resistance (ESCR), low shrinking, and high service temperature.
A coating composition comprising a multimodal ethylene polymer is known through WO 97/03139. This multimodal ethylene polymer contains from 80 to 100% by weight of ethylene repeating units and from 0 to 20% by weight of C3-C10 α-olefin repeating units, has a density of between 0.915 g/cm3 and 0.955 g/cm3, and is a blend of at least a first ethylene polymer having a first average molecular weight and a first molecular weight distribution and a second ethylene polymer having a second molecular weight, which is higher than said first molecular weight, and a second molecular weight distribution, said blend having a third molecular weight and a third molecular weight distribution. The multimodal ethylene polymer is said to have good processability and low shrinkage as well as superior environmental stress cracking resistance. Preferentially, the coating composition has an environmental stress cracking resistance (ESCR, F20) (ASTM D 1693/A, 10% Igepal) of at least 100 h, more preferentially at least 500 h, yet more preferentially at least 1000 h and most preferentially 2000 h. For the preparation of the multimodal ethylene polymer any kind of ethylene polymerisation catalyst, such as a chromium catalyst, a Ziegler-Natta catalyst or a group 4 transition metallocene catalyst may be used. No examples of metallocene catalysts are given, however, and all the multimodal ethylene polymers in the examples of WO 97/03139 have been prepared with the use of Ziegler-Natta catalysts.
In this connection WO 01/62847 should also be mentioned. This document relates to a process for coating a substrate by extrusion coating on the substrate 90-100% by weight of a bimodal polyethylene composition that has been polymerised in the presence of a single site catalyst (SSC), and up to 10% by weight of high pressure PE-LD. The coating composition has a melt flow rate (MFR2) of 5 g/10 min or more. The specification indicates that the coating has a thickness of about 10-200 μm and that it is applied on thin substrates like paper, cardboard, polymer films, or aluminium foil.
WO 00/34341 discloses a heterogeneous olefin polymerization catalyst comprising a porous particulate support material having impregnated therein a metallocene catalyst the metal atom whereof is bound to an amide group which is not bound to an η-ligand of the metallocene, or the reaction product of said metallocene catalyst and a cocatalyst. No mention is made of coating compositions or the preparation thereof, particularly not steel pipe coating compositions.
Although the coating compositions according to the prior art such as WO 97/03139 are sufficient for many purposes there is always a need for still more improved coating compositions, especially with regard to the critical property of environmental stress cracking resistance (ESCR).